The instant invention relates to a regulated drawing frame of the textile industry, i.e. a drawing frame in which draft is controlled or controllably adjustable. The concept of control comprises in this case the application of controls or of a multi-looped control system.
In this invention fiber slivers are processed and the thickness of the end-product must be uniform. It is the task of the control system to recognize a change in fiber sliver thickness and to bring it to the desired uniform thickness by drafting.
The thickness signals are detected at the measuring station before the inlet of the drawing frame. All along the subsequent course taken by the measured points in the fiber sliver, and up to the place where drafting occurs, the appertaining measuring signal is buffer-stored with a delay time. At the end of this delay time the control system intervenes immediately as a function of the deviation of the fiber sliver thickness. This onset of this control application is called the regulation onset point.
The problem in this case is that the regulation onset point must occur neither too early nor too late with respect to the onset of drafting because this would result in faulty drafting. Similarly, the intensity of regulation, i.e. the amplification, may be neither too low nor too high.
In practice, influences attributable to the machine or environmental influences are the cause that the drafting point cannot be determined precisely, and therefore errors occur in determining the regulation onset point and the intensity of regulation.
When erratic fluctuations of fiber sliver thickness occur, for example as a result of needle impulses exceeding the tolerance limits, the mechanical components are unable to follow quickly enough in driving the drawing frame rollers because of their inertia. Complete compensation for the variations in thickness is hardly possible in this case. The problem is aggravated by the existing need to increase the speed of the fiber sliver from an average of 500 m per minute to 800 m per minute and more.
Thickness fluctuations which increase very slowly over time are yet another extreme case. Here the reaction of regulation is also insufficient.
DE-OS 36 19 248 proposes that a correction value be determined for the delay period as a function of the steepness or the relative magnitude of mass fluctuation. The result is a shorting of the delay time as a function of the steepness and magnitude of the mass fluctuation. This solution has the disadvantage that the result of regulation cannot be checked. This is a disadvantage insofar as the correction made can be subject to influences attributable to the machine or by environmental influences.
The solution according to EP 412 448 proposes the utilization of a multi-looped control system on the drawing frame, whereby the measuring signal is detected and evaluated after the drawing frame output. The proposed solution here is to ascertain the result of controlled drafting change through supervision along the drawing frame course, to re-enter it into the same control system and to evaluate it in an optimization process broken down into low-frequency and high-frequency portions. The setting magnitude Y which is optimized by the main control is thus used as a setting value for the controller 8.2 of the drive of the main drafting zone 12 (EP 412 488, page 12, lines 12 -15). This solution is mainly based on the utilization of the measured values in order to always optimize the setting value. The clear drawback in this method is the fact that correction values to be used to set the setting values are not processed independently by the control system, and are therefore not free from influences.
To be able to detect changes in the regulation with certainty and independently of the regulation, the "sliver test" was conducted in the past. The "sliver test" is conducted by spot-checking and manually determining the correct levelling out of fluctuations in fiber sliver thickness. A test sliver is produced. The operator adds an individual sliver segment to the presented slivers or produces a limited sliver interruption through sliver breakage. The length of this created fiber sliver is cut out and its actual sliver thickness is determined by weighing (see instruction manual of the RIETER Spinning Systems, drawing frame RSB 851, SB 851, point 4.5.6, edition 8/1990). It is thus impossible to avoid an interruption of production counted in minutes. This is a considerable disadvantage in continuous production at high production speeds.